Membrane pumps for pumping blood are well known in the art. For instance from European patent 1 072 278 and WO-A1-2005/021078 single-lumen membrane blood pumps are known which include a displacement device in the form of a generally rigid housing bounding an inner space divided in two compartments by a flexible membrane. In operation, one compartment contains blood and the other compartment contains a gas (e.g. ambient air). This known displacement device has a socket to which a single-lumen catheter is connected. The single-lumen catheter has an outlet passage at a distal end thereof and inlet passages proximally spaced from the outlet for transporting blood through the catheter from the inlet to the outlet over the distance between the inlet and the outlet. The gas compartment has an opening for connection to a conduit communicating with a drive unit. The drive unit generates under pressure and overpressure (relative to the current pressure in the lumen and the blood compartment), thereby alternatingly removing gas from the gas compartment and pressing gas into the gas compartment, respectively. During the aspiration phase, the under pressure draws the membrane towards a first side of the shell, aspiring blood via the inlet passages and also, to some extent, via the outlet passage of the single-lumen catheter into the membrane pump. This is followed by the ejection phase, when the pressure pushes the membrane back from the first side of the shell towards a second side of the shell opposite of the first side, thereby expelling blood out of the shell and into the catheter lumen. When blood is expelled, the orientation of the flow past the inlet opening causes no or relatively little of the blood to flow out via the inlet opening, so that alternatingly aspiring and ejecting blood in a sequential mode, results in a net pumping effect. In such a single-lumen pump, in which blood flows into and out of a compartment, the residence time of some of the blood in the blood compartment can become quite long which entails a risk of clot formation.
From WO 00/03754 a membrane pump is known which also includes an outer shell with a membrane dividing the inner space in a blood compartment and a gas compartment. In this membrane pump two sockets, each containing a one-way valve, allow blood to flow in and out of the blood compartment and each communicate with a lumen of a double lumen portion of the catheter, the lumen joining into a single-lumen in a position spaced from the sockets. The sockets are each provided with a one-way valve, the valves being arranged such that one of the sockets allows blood to flow into the membrane pump only while the other socket allows blood to flow out of the blood compartment only. According to this document, turbulence in the blood flow and exposure of blood components to mechanical stress are minimized because the blood path is at least partially circulating, so stagnation of the blood flow is prevented. However, at the valves, local stagnation of the flow, turbulence and exposure of blood components to mechanical stress can be expected.
In both of the above-discussed membrane pumps, the membrane is mounted in-between two shell parts forming the outer shell. As blood has the ability to clot easily, all edges and sharp corners need to be rounded in order to avoid blood damage and clotting. This rounding is performed by smoothening the corners and edges. The transition from the membrane to the rigid outer shell is especially vulnerable for clotting. This requires several coating processes to make sure the transition is completely smoothened. In addition, the transitions of the top shell to the socket/sockets need to be smoothened. For these reasons, the manufacturing of membrane pumps requires several time consuming steps. This makes membrane pumps relatively expensive, which is particularly disadvantageous because sterility requirements impose that cardiac assist membrane pumps are used as disposable items only.